The invention relates to the reduction of speckle in optical systems. Specifically, this invention proposes an electro-optic modulator that alters phase wavefronts of an incident beam thereby reducing spatial or temporal coherence in a light beam.
Projection display systems for the display of video images are well-known in the prior art. Typically, these systems have taken the basic form of a white light source, most notably a xenon arc lamp, illuminating one or more light valves or spatial light modulators with appropriate color filtering to form the desired image, the image being projected onto a viewing screen.
Lasers have been known to be an attractive alternative light source to arc lamps for projection displays. One potential advantage is a wider color gamut featuring very saturated colors. Laser illumination offers the potential for simple, low-cost efficient optical systems, providing improved efficiency and higher contrast when paired with some spatial light modulators. One disadvantage of lasers for projection display has been the lack of a cost-effective laser source with sufficient power at visible wavelengths.
Laser sources appropriate for display systems are becoming available and are known in the prior art. Two examples are produced by JenOptik and Lumera Laser, GmbH, and are mode-locked, diode-pumped, solid-state lasers, each with a nonlinear-optical system featuring an optical parametric oscillator (OPO) to simultaneously generate red, green, and blue light. This system has been disclosed by Wallenstein in U.S. Pat. No. 5,828,424, issued Oct. 27, 1998 and U.S. Pat. No. 6,233,025 issued May 15, 2001; and by Nebel in U.S. Pat. No. 6,233,089, issued May 15, 2001. Another example disclosed by Moulton in U.S. Pat. No. 5,740,190, issued Apr. 14, 1998 is developed by Q-Peak and is a Q-switched DPSS laser with an OPO system to simultaneously generate red, green, and blue light.
Two-dimensional spatial light modulators can be used in laser display systems. Examples include reflective liquid crystal modulators such as the liquid-crystal-on-silicon (LCOS) modulators available from JVC, Three-Five, Aurora, and Philips, and micromirror arrays such as the Digital Light Processing (DLP) chips available from Texas Instruments. Advantages of two-dimensional modulators over one-dimensional array modulators and raster-scanned systems are the lack of a need for vertical scanning systems, absence of streak artifacts due to non-uniformities in the modulator array, and immunity to laser noise at frequencies much greater than the frame refresh rate (xe2x89xa7120 Hz). A further advantage of two-dimensional spatial light modulators is the wide tolerance for reduction of the spatial coherence of the illuminating beam.
One major disadvantage of the use of lasers in projection displays is the speckle effect. Speckle arises due to the high degree of coherence (both spatial and temporal) inherent in most laser sources. Speckle produces a noise component in the image, as a granular structure, which both degrades the actual sharpness of the image and annoys the viewers. As such, the speckle problem, as well as the historical lack of appropriate laser sources, has inhibited the development of marketable laser-based display systems.
The prior art is rich in ways of attempting to reduce speckle. One common approach is to reduce the temporal coherence of a laser beam by broadening the linewidth. Ultrafast lasers may help, but are not believed to reduce speckle to the required degree. Other approaches to reducing the temporal coherence are to split the illuminating wavefront into beamlets and delay them relative to each other by longer than the coherence time of the laser. Variations on this basic idea have been disclosed in U.S. Pat. No. 3,633,999 issued Jan. 11, 1972 to Buckles; U.S. Pat. No. 4,511,220 issued Apr. 16, 1985 to Scully; U.S. Pat. No. 4,521,075 issued Jun. 4, 1985 to Obenschain et al.; U.S. Pat. No. 4,619,508 issued Oct. 28, 1986 to Shibuya et al.; U.S. Pat. No. 5,153,773 issued Oct. 6, 1992 to Muraki et al.; U.S. Pat. No. 5,224,200 issued Jun. 29, 1993 to Rasmussen et al.; U.S. Pat. No. 5,363,170 issued Nov. 8, 1994 to Muraki; and U.S. Pat. No. 6,011,643 issued Jan. 4, 2000 to Wunderlich et al. A disadvantage of this approach is the complication it introduces to the optical system to implement a sufficient number of different beam paths.
Dynamically varying the speckle pattern is another way of reducing the visibility of the speckle pattern. One way of doing this is to vibrate or otherwise alter the viewing screen. Vibrating a screen or multiple screens have been disclosed in U.S. Pat. No. 4,033,683 issued Jul. 5, 1977 to Tancredi; U.S. Pat. No. 4,143,943 issued Mar. 13, 1979 to Rawson; and U.S. Pat. No. 5,272,473 issued Dec. 21, 1993 to Thompson et al. Active screens with dynamically varying diffusion properties have been disclosed in U.S. Pat. No. 3,650,608 issued Mar. 21, 1972 to Baker; U.S. Pat. No. 6,092,900 issued Jul. 25, 2000 to Diedrich et al.; and U.S. Pat. No. 6,122,023 issued Sep. 19, 2000 to Chen et al. A disadvantage to this approach is the expense of the active screens and the need to engineer a special screen for each theater. A cost-effective despeckling solution implemented within the projector is desired.
Within the projector system, a number of approaches have been attempted to dynamically vary the speckle pattern. One concept employs coupling the laser light into a multimode optical fiber and vibrating the fiber to cause mode-scrambling. This approach is found in U.S. Pat. No. 3,588,217 issued Jun. 28, 1971 to Mathisen, and U.S. Pat. No. 4,011,403 issued Mar. 8, 1977 to Epstein et al. Disadvantages associated with this approach are insertion losses due to coupling into the fiber, and loss of polarization, which is required with some light valves.
Another despeckling concept utilizes vibration or motion of the optical components within the projector to vibrate the image, such as rotating lenslet arrays disclosed in U.S. Pat. No. 3,851,515 issued Dec. 3, 1974 to Hautau and U.S. Pat. No. 6,081,381 issued Jun. 27, 2000 to Shalapenok et al., and vibrating an entire image as shown in published application WO 00/65401 published Nov. 2, 2000. The former has the disadvantage of only allowing circular illumination patterns, while the latter has a deleterious effect on image quality.
Finally, another family of despeckling solutions features a diffusing element that is moved or vibrated within the projector system. Typically, this is done at an intermediate image plane, as disclosed in U.S. Pat. No. 4,035,068 issued Jul. 12, 1977 to Rawson; U.S. Pat. No. 4,201,473 issued May 6, 1980 to Domenicali et al.; U.S. Pat. No. 4,256,363 issued Mar. 17, 1981 to Briones; U.S. Pat. No. 5,164,848 issued Nov. 17, 1992 to Firth et al.; U.S. Pat. No. 5,534,950 issued Jul. 9, 1996 to Hargis et al.; and U.S. Pat. No. 6,154,259 issued Nov. 28, 2000 to Hargis et al. A disadvantage of this approach is that the diffusion must occur precisely at the image plane or a softening of the image will occur. Also, the projection lens is complicated by the requirement to provide an intermediate image plane.
It has been proposed to dynamically vary the speckle pattern by dynamically diffusing the laser beam in the illumination path of the device. This approach has been disclosed by vanLigten in U.S. Pat. No. 3,490,827, issued Jan. 20, 1970, in which a diffuser is rotated in the focus of a beam expander. Florence discloses, in U.S. Pat. No. 5,313,479, issued May 17, 1994, illuminating a light valve through a rotating diffuser. These approaches have the disadvantage of not being adaptable to efficient illumination of a rectangular spatial light modulator. Butterworth in Patent U.S. Pat. No. 6,005,722, issued Dec. 21, 1999, discloses a system in which a variable-thickness plate is rotated in the illumination of a light pipe homogenizer. Light pipe homogenizers, though, require either a large numerical aperture or a substantial length to achieve optimum uniformity.
Another disadvantage of using a laser as a light source in an image projector is the susceptibility of interference or the occurrence of diffraction artifacts in the light valve. This is especially true of liquid crystal modulators, wherein the thin-film structure can result in fringes in the image due to non-uniformities in the film layers. Diffraction artifacts arise from illuminating a grid electrode pattern in the light modulator with a highly coherent beam of light.
There is a need therefore for an optical device that alters the wavefront phase of a light beam or reduces coherence, such that when the device is incorporated into a display system, speckle and other coherence artifacts are reduced.
The need is met according to the present invention by providing a method of reducing the appearance of speckle resulting from a coherent light beam, including the steps of: providing an electro-optic device having an electro-optic substrate, an electrode array arranged on the surface of the electro-optic substrate, at least one electrode located on the opposite surface of the electro-optic substrate, and means for applying voltage to the electrodes to generate a variation in a refractive index profile within the substrate; and directing the light beam through the electro-optic device while applying voltage to the device, whereby the coherence of the light beam is reduced, thereby reducing the appearance of speckle through phase front modulation.